APPLICATIONS OF DIFFERENTIATION

1. 4 APPLICATIONS OF DIFFERENTIATION

2. Summary If f’(x) > 0 on an interval then f is increasing on that interval. If f’(x) < 0 on an interval then f is decreasing on that interval. If f’(c)= 0 and f’(x) > 0 for x<c and f’(x)<0 for x >c , then c is a local maximum. If f’(c)= 0 and f’(x) < 0 for x<c and f’(x)>0 for x >c , then c is a local minimum.

3. SECOND DERIVATIVE TEST Suppose f’’ is continuous near c. • If f’(c) = 0 and f’’(c) > 0, then f has a local minimum at c. • If f’(c) = 0 and f’’(c) < 0, then f has a local maximum at c.

4. APPLICATIONS OF DIFFERENTIATION Many applications of calculus depend on our ability to deduce facts about a function f from information concerning its derivatives.

5. APPLICATIONS OF DIFFERENTIATION 4.3How Derivatives Affect the Shape of a Graph In this section, we will learn: How the derivative of a function gives us the direction in which the curve proceeds at each point.

6. DERIVATIVES AND GRAPH SHAPE As f’(x) represents the slope of the curve y = f(x) at the point (x, f(x)), it tells us the direction in which the curve proceeds at each point. • Thus, it is reasonable to expect that information about f’(x) will provide us with information about f(x).

7. WHAT DOES f’ SAY ABOUT f ? To see how the derivative of f can tell us where a function is increasing or decreasing, look at the figure. • Increasing functions and decreasing functions were defined in Section 1.1

8. WHAT DOES f’ SAY ABOUT f ? Between A and B and between C and D, the tangent lines have positive slope. So, f’(x) > 0.

9. WHAT DOES f’ SAY ABOUT f ? Between B and C, the tangent lines have negative slope. So, f’(x) < 0.

10. WHAT DOES f’ SAY ABOUT f ? Thus, it appears that f increases when f’(x) is positive and decreases when f’(x) is negative. • To prove that this is always the case, we use the Mean Value Theorem.

11. INCREASING/DECREASING TEST (I/D TEST) • If f’(x) > 0 on an interval, then f is increasing on that interval. • If f’(x) < 0 on an interval, then f is decreasing on that interval.

12. I/D TEST Proof a Let x1 and x2 be any two numbers in the interval with x1 < x2. According to the definition of an increasing function, we have to show that f(x1) < f(x2).

13. I/D TEST Proof a (Equation 1) Since we are given that f’(x) > 0 f(x2) – f(x1) = f’(c)(x2 – x1) We know that f’(c) > 0 and (x2 – x1) > 0 Then f’(c)(x2 – x1) > 0 Hence f(x2) – f(x1) > 0 f(x2) > f(x1)

14. I/D TEST Proof a (Equation 1) If f’(x) < 0 for every x f(x2) – f(x1) = f’(c)(x2 – x1) We know that f’(c) < 0 and (x2 – x1) > 0 Then f’(c)(x2 – x1) < 0 Hence f(x2) – f(x1) < 0 f(x2) < f(x1)

15. I/D TEST Example 1 Find where the function f(x) = 3x4 – 4x3 – 12x2 + 5 is increasing and where it is decreasing.

16. I/D TEST Example 1 f’(x) = 12x3 - 12x2 - 24x = 12x(x – 2)(x + 1) • To use the ID Test, we have to know where f’(x) > 0 and where f’(x) < 0. • This depends on the signs of the three factors of f’(x)—namely, 12x, x – 2, and x + 1.

17. I/D TEST Example 1 We divide the real line into intervals whose endpoints are the critical numbers -1, 0, and 2 and arrange our work in a chart.

18. I/D TEST Example 1 A plus sign indicates the given expression is positive. A minus sign indicates it is negative. The last column gives the conclusion based on the I/D Test.

19. I/D TEST Example 1 For instance, f’(x) < 0 for 0 < x < 2. So, f is decreasing on (0, 2). • It would also be true to say that f is decreasing on the closed interval.

20. I/D TEST Example 1 The graph of f confirms the information in the chart.

21. WHAT DOES f’ SAY ABOUT f ? Recall from Section 4.1 that, if f has a local maximum or minimum at c, then c must be a critical number of f (by Fermat’s Theorem). • However, not every critical number gives rise to a maximum or a minimum. • So, we need a test that will tell us whether or not fhas a local maximum or minimum at a critical number.

22. WHAT DOES f’ SAY ABOUT f ? You can see from the figure that f(0) = 5 is a local maximum value of f because f increases on (-1, 0) and decreases on (0, 2). • In terms of derivatives, f’(x) > 0 for -1 < x < 0 and f’(x) < 0 for 0 < x < 2.

23. WHAT DOES f’ SAY ABOUT f ? In other words, the sign of f’(x) changes from positive to negative at 0. • This observation is the basis of the following test.

24. FIRST DERIVATIVE TEST Suppose that c is a critical number of a continuous function f. a. If f’ changes from positive to negative at c, then f has a local maximum at c.

25. FIRST DERIVATIVE TEST b. If f’ changes from negative to positive at c, then f has a local minimum at c.

26. FIRST DERIVATIVE TEST c. If f’ does not change sign at c—for example,if f’ is positive on both sides of c or negative on both sides—then f has no local maximum or minimum at c.

27. FIRST DERIVATIVE TEST The First Derivative Test is a consequence of the I/D Test. • For instance, in (a), since the sign of f’(x) changes from positive to negative at c, f is increasing to the left of c and decreasing to the right of c. • It follows that f has a local maximum at c.

28. FIRST DERIVATIVE TEST It is easy to remember the test by visualizing diagrams.

29. WHAT DOES f’ SAY ABOUT f ? Example 2 Find the local minimum and maximum values of the function fin Example 1.

30. WHAT DOES f’ SAY ABOUT f ? Example 2 From the chart in the solution to Example 1, we see that f’(x) changes from negative to positive at -1. • So, f(-1) = 0 is a local minimum value by the First Derivative Test.

31. WHAT DOES f’ SAY ABOUT f ? Example 2 Similarly, f’ changes from negative to positive at 2. • So, f(2) = -27 is also a local minimum value.

32. WHAT DOES f’ SAY ABOUT f ? Example 2 As previously noted, f(0) = 5 is a local maximum value because f’(x) changes from positive to negative at 0.

33. WHAT DOES f’’ SAY ABOUT f ? The figure shows the graphs of two increasing functions on (a, b).

34. WHAT DOES f’’ SAY ABOUT f ? Both graphs join point A to point B, but they look different because they bend in different directions. • How can we distinguish between these two types of behavior?

35. WHAT DOES f’’ SAY ABOUT f ? Here, tangents to these curves have been drawn at several points.

36. CONCAVE UPWARD In the first figure, the curve lies above the tangents and f is called concave upward on (a, b).

37. CONCAVE DOWNWARD In the second figure, the curve lies below the tangents and g is called concave downward on (a, b).

38. CONCAVITY—DEFINITION If the graph of f lies above all of its tangents on an interval I, it is called concave upward on I. If the graph of f lies below all of its tangents on I, it is called concave downward on I.

39. CONCAVITY The figure shows the graph of a function that is concave upward (CU) on the intervals (b, c), (d, e), and (e, p) and concave downward (CD) on the intervals (a, b), (c, d), and (p, q).

40. CONCAVITY Let’s see how the second derivative helps determine the intervals of concavity.

41. CONCAVITY From this figure, you can see that, going from left to right, the slope of the tangent increases. • This means that the derivative f’ is an increasing function and therefore its derivative f” is positive.

42. CONCAVITY Likewise, in this figure, the slope of the tangent decreases from left to right. So, f’ decreases and therefore f’’ is negative. • This reasoning can be reversed and suggests that the following theorem is true.

43. CONCAVITY TEST • If f’’(x) > 0 for all x in I, then the graph of f is concave upward on I. • If f’’(x) < 0 for all x in I, then the graph of f is concave downward on I.

44. INFLECTION POINT—DEFINITION A point P on a curve y = f(x) is called an inflection pointif f is continuous there and the curve changes from concave upward to concave downward or from concave downward to concave upward at P.

45. INFLECTION POINT For instance, here, B, C, D, and P are the points of inflection. • Notice that, if a curve has a tangent at a point of inflection, then the curve crosses its tangent there.

46. INFLECTION POINT In view of the Concavity Test, there is a point of inflection at any point where the second derivative changes sign.

47. WHAT DOES f’’ SAY ABOUT f ? Another application of the second derivative is the following test for maximum and minimum values. • It is a consequence of the Concavity Test.

48. SECOND DERIVATIVE TEST Suppose f’’ is continuous near c. • If f’(c) = 0 and f’’(c) > 0, then f has a local minimum at c. • If f’(c) = 0 and f’’(c) < 0, then f has a local maximum at c.

49. SECOND DERIVATIVE TEST For instance, (a) is true because f’’(x) > 0 near c, and so f is concave upward near c. • This means that the graph of f lies above its horizontal tangent at c, and so f has a local minimum at c.

50. WHAT DOES f’’ SAY ABOUT f ? Example 6 Discuss the curve y = x4 – 4x3with respect to concavity, points of inflection, and local maxima and minima. Use this information to sketch the curve.